bearing insulation ressitance limits for INSTALLED motors

quote:

Does anyone know typical ranges of shaft voltages and how much current is required to cause damage? (might use this to guesstimate min acceptable resistance)

Talking about the non-vfd motor, the induced voltage must be caused by asymmetry of the flux. In case of the 2-pole motor (which is the easiest to imagine) it would be the difference between the flux going on one side of the shaft compared to the other side (of the shaft). Hence the voltage must be substantially lower than the turn-to-turn voltage (shaft, bearings, endbels and frame of the motor constitute a one turn). One can imagine a 1000 hp motor with turn-to-turn about 25 Volts. From that I arrive at voltages very likely below 1 Volt. I have never measured it, but the resistance of 30 kOhms seems to be plenty good to me.
jank

Pete,
What I have come across: typical ranges of shaft voltages are: 100 mVac .. 1.0 Vac, frequency 1* .. 6* Fline and/or 1*RBPF depending on elec/mech/magnetic asymmetry in design of asynchronous, synchronous or dc machine, kW .. MW range.
Shaft short-circuit current may range from 10 mA to 10 A, same frequency, also depending on design.
Since frequency range is pretty low, say 50 hz .. 60 hz up to 1 khz .. 2 khz an actual (Al2O3 ceramics) insulation resistance of 30 kohm is enough to reduce shaft circulating current to much less than 1 mA and this is I conguesstimatesider as safe.

As far as I know there are no standards for installed machines.

But be carefull when modern vfd is involved. For very high spikes from the rapid dU/dt switching the insulator may behave as a capacitor and will conduct current. This type of shaft current ranging from 100 khz into Mhz is, I am almost certain, not yet covered by standards. I have already seen several insulated bearings that are not insulating (vfd applications only) resulting in damaged bearings because of electrical current.

Q: can you tell us how you measure that insulation resistance? With machine running, uncoupled, no-load, using shaft riders, what voltage you apply?

Regards,
Arie Mol, NL
Rotating Equipment Consultant

Thanks Jan and Arie for your responses.

I will mention there is another thread I started on another forum addressing the same question.

http://www.eng-tips.com/viewthread.cfm?qid=208967&page=1

In there you'll see among other things a link to an article on ths subject
http://www.netaworld.org/files/ItemFileA340.pdf
Still doesn't shed a tremendous amount of light.

The motors I am interested in are not vfd driven.

We test bearing insulation resistance with the machine secured and motor uncoupled using a 500vdc megger. (You'll note this matches the standards cited in my original post). We check from shaft to frame (the uninsulated portion of the frame which excludes lower bearings housing... in this case lower bearing housing is in contact with lower beianrg but inulated from lower bearing bracket and the rest of the frame). It works for this particular motor because both upper and lower bearings are insulated.

I do notice that several large reputable OEM's has given very low limits for machines with sliding bearings. For example Westinghouse identifies 3 kohms minimum for a group of our critical sliding bearing motors. I am not sure I can extend these values to rolling bearing motors. It seems to me that sliding bearing motors have a built in insulator at the bearings in the form of an oil wedge while running... not the case for rolling bearings.

Pete,
Two comments:
1)
<qoute>It works for this particular motor because both upper and lower bearings are insulated.<unquote>.
Yes, but what to do when only one bearing is insulated, then it does not work right? Not many motors have both bearing insulated!
My trick: Under normal (any) running conditions, short-circuit the nde shaft to ground with a shaft rider and a 2.5 mm2 cable. Than measure the current with a clip-on ammeter. Also measure the shaft voltage over both shaft ends. Good old Ohm's law tells you the impedance of the bearing insulation (= ceramic bearing insulation + oil film insulation) of de end bearing arrangement. If current is zero then bearing isolates perfectly.
Analogous for short-circuiting the de shaft.
2)
<quote>It seems to me that sliding bearing motors have a built in insulator at the bearings in the form of an oil wedge while running... not the case for rolling bearings.<unquote>.
Yes, but rolling element bearings have an oil film too!! What else would be the lubrication mechanism?
Above mentioned test methode will reveal that clean oil in rolling element bearing provides an insulator. Contaminated oil (carbon, steel wear particles) will show up impedances in MOhm .. kOhm range or even lower.

Regards,
Arie Mol